The present invention relates to a method for reciprocating a vibratory conveyor to transport products, and more particularly to a method for reciprocating the conveyor variably to achieve higher degrees of control and versatility with respect to product movement than have previously been possible.
Vibratory conveyors are used industrially for moving products of different shapes and weights from one location to another. For example, in the packaging of fragile food products such as potato chips or cookies, the food product is received from a central location, such as a cooking oven, and conveyed to a plurality of work stations having packaging machines. The processing of produce, i.e. fruits and vegetables, similarly requires the handling of fragile food products. Vibratory conveyors are especially useful in such applications because such fragile food products may not readily be transported in other ways without damaging the products.
Perhaps the most common vibratory conveyor design features an elongate conveyor pan mounted on a frame by inclined leaf springs or vertical coil springs which cause the pan to reciprocate not only in its lengthwise direction but also normal to its lengthwise direction. Products resting on the pan are thrown upward and forward in the direction of conveyance with each cycle of the drive system, as exemplified by the conveyors shown in U.S. Pat. Nos. 2,094,787, 4,371,800, 4,378,064, and 4,921,090. A significant drawback of this type of vibratory conveyor is damage to the products as they bounce in response to the upward component of the reciprocating pan motion. Also, inherent in this type of conveyor is the inability to provide product flow in opposite directions along the conveyor, because the conveyor pan is mounted to throw products forward in only one direction. Moreover, changing the average velocity of product flow requires a change in reciprocating frequency which may place the frequency undesirably close to the resonant frequency of the conveyor support structure and/or building housing the conveyor. Further, products having different frictional resistances to slip relative to the conveyor pan, due to differences in product batches, temperatures, produce skin conditions or other variables, travel along the conveyor at significantly different average velocities. The inability of the conveyor to compensate for these differences to maintain a substantially constant average product velocity complicates the processing of products at work stations by uncontrollably allowing variations in volumetric flow.
Other previous vibratory conveyor systems have substantially horizontal vibratory motions for moving products along the conveyor, such as the conveyor system manufactured under the trademark SlipStick by Triple/S Dynamics, Inc. of Dallas, Tex. The SlipStick conveyor pan is suspended from pendulum-type hangers which permit it to reciprocate only substantially horizontally, with no significant component of reciprocation normal to the horizontal direction. The conveyor is driven by a uniform-speed motor connected to a mechanical gearing and eccentric weight system that provides a conveyor reciprocating motion pattern having unsymmetrical rates of motion in its opposite directions. The forward motion of the conveyor generally has lesser acceleration and velocity than does the rearward motion, causing the products to move with less slippage relative to the conveyor when the conveyor is moving forwardly than when it is moving rearwardly. This type of motion can automatically compensate for differences in resistance to slip to maintain a constant average product velocity, but only for a single average velocity and conveyor loading. If it is desired to change these parameters while maintaining such compensation for differences in resistance to slip, reconfiguration of the mechanical gearing and weight system is required. Merely changing the average velocity of product flow, without maintaining such compensation for differences in resistance to slip, can be accomplished without mechanical reconfiguration by raising or lowering the uniform speed of the motor, but such changed product velocities are relatively unstable, particularly for larger magnitudes of change, and require a change in reciprocating frequency which may place the frequency undesirably close to the resonant frequency of the conveyor support structure and/or building housing the conveyor. Products can also be moved at the same average velocity in the reverse direction with such a system by reversing the motor, but the direction cannot be changed without stopping the motor and restarting the reciprocating motion in the opposite direction which consumes considerable time.
Many previous vibratory conveyors have no sensor feedback system for automatically variably controlling power to the drive motor to maintain any particular conveyor motion pattern independently of variables such as conveyor loading. For example, the aforementioned SlipStick conveyor, although having a sensor for detecting both excessive and insufficient amplitudes of conveyor reciprocation, merely automatically turns the conveyor off if either of these conditions is detected. On the other hand, systems such as those shown in U.S. Pat. Nos. 4,441,060 and 5,127,512 sense conveyor motion and adjust power in response thereto to ensure maintenance of a desired motion pattern. However, such systems do not use feedback to drive a motor with a cyclical nonuniform motion to maintain a precise conveyor motion pattern with unsymmetrical rates of motion in opposite directions. Accordingly, such systems cannot maintain a precise average product velocity despite differences in resistance to slip, nor provide instantaneous conveyor reversibility, nor provide the same precise average product velocity at different conveyor inclinations and reciprocating frequencies.